This application claims priority to application No. 2002-0005018, filed in the Korean Intellectual Property Office on Jan. 29, 2002 the disclosure of which is incorporated hereinto by reference.
(a) Field of the Invention
The present invention relates to a cathode ray tube with a tension mask wherein tensional strength is applied in uni-axial or bi-axial directions, and more particularly, to a tension mask cathode ray tube with an inner shield capable of minimizing variation in the landing of the electron beam due to geomagnetism.
(b) Description of the Related Art
Generally, a cathode ray tube is a display device wherein three electron beams are scanned on a phosphor screen to thereby display a desired picture image. The route of each electron beam is varied with the axes of north and south poles of the earth, due to geomagnetism. The electron beam is influenced by a purity characteristic, a raster position, and a convergence characteristic.
The geomagnetic field is divided into a vertical force (the vertical geomagnetic field) perpendicular to the earth""s surface, and a horizontal force (the horizontal geomagnetic field) parallel to the earth""s surface. The geomagnetic field involves different values depending upon the location thereof on the earth. With the cathode ray tube, the movement of the electron beam due to the horizontal geomagnetic field may be shown to be divided into an NS movement factor and an EW movement factor, with respect to the cathode ray tube axis.
The NS movement refers to the movement of the electron beam due to the horizontal geomagnetic field corresponding to the tube axis of the cathode ray tube, while the EW movement refers to the movement of the electron beam due to the horizontal geomagnetic field perpendicular to the tube axis of the cathode ray tube.
The amount of variation in the landing position of the electron beam displayed at the screen under the influence of the geomagnetic field may be shown to be divided into a horizontal component and a vertical component.
Both with a shadow mask for a color picture tube for public use with a longitudinal slot in the vertical direction, and a shadow mask for a color display tube for industrial use with dot-type holes, the electron beam becomes distant from the designated slot or hole due to the horizontal movement component thereof. Accordingly, it is critical to prohibit the horizontal movement component.
Typically an inner shield is mounted within the cathode ray tube to reduce the amount of variation in the landing position of the electron beam due to the geomagnetic field. In the case of a cathode ray tube with a mask formed by way of press-forming (referred to hereinafter as the formed mask cathode ray tube), the inner shield is fabricated using a high magnetic permeable material, thereby reducing the movement scale of the electron beam due to the geomagnetic field.
As shown in FIG. 1A, in the case a cylindrical shield 110 is made using two materials 112 bearing the same magnetic permeability, the magnetic force line of the geomagnetic field passes through the inside of the materials 112 as indicated by the arrows. Consequently, the internal magnetic field of the shield 110 is stabilized.
By contrast, as shown in FIG. 1B, in the case a cylindrical shield 110xe2x80x2 is made using a high magnetic permeability material 112 and a low magnetic permeability material 114 (the permeability thereof being {fraction (1/10)} of the high magnetic permeability material), leakage of the magnetic field occurs at the interface 116 between the two materials 112 and 114 as indicated by the arrows. Consequently, the internal magnetic field of the shield 110xe2x80x2 becomes non-uniform while reducing the shielding effect.
In the case of a formed mask cathode ray tube, the initial magnetic permeability xcexc0.35 of the mask is about 600, and the maximum magnetic permeability xcexcmax thereof is about 3200. The material for the mask frame used to hold the formed mask has an initial magnetic permeability xcexc0.35 of 800 or more, and the maximum magnetic permeability xcexcmax thereof is 4000-8000.
The initial magnetic permeability is a value measured at a magnetic flux density of 350 mG.
Therefore, when the material for the inner shield has the same magnetic permeability as the mask frame, as shown in FIG. 1A, the magnetic force flows smoothly while increasing the shielding efficiency, as with the case where the two materials 112 bearing the same magnetic permeability are coupled to each other. In this way, the movement scale of the electron beam is reduced.
Korean Patent Publication Nos. 1998-077085 to 077088 and 1999-026171 disclose a method of improving the magnetic permeability of the inner shield material by way of heat treatment.
However, with a cathode ray tube using a tension mask (referred to hereinafter as the tension mask cathode ray tube), as the tension mask and the mask frame for holding the mask must bear a high force, the magnetic permeability of the material for the tension mask and the mask frame is less than that of the formed mask cathode ray tube.
Table 1 illustrates the magnetic characteristics of a usual tension mask and a mask frame. The magnetic characteristics are measured under a condition such that the tension mask and the mask frame are blackened at 460xc2x0 C., the tension mask is tensioned in the uni-axial or bi-axial directions with a force of 15 kgf/mm2 or more, and they are mounted within the cathode ray tube. In Table 1, Hc indicates the coercive force, and Br indicates the residual magnetic flux density.
As illustrated in Table 1, the tension mask and the mask frame exhibit a magnetic permeability xcexc of about 20% of the relevant parts of the formed mask cathode ray tube.
Accordingly, when a high magnetic permeable inner shield is mounted to a tension mask cathode ray tube, the phenomenon illustrated in FIG. 1B occurs. That is, with the use of a high magnetic permeable inner shield, a low magnetic permeable tension mask, and a mask frame, the shielding effect of the inner shield may be satisfactorily produced, but variations in the local magnetism distribution occur due to leakage of the magnetic field at the portion where the inner shield is welded to the frame. Consequently, the movement scale of the electron beam is increased.
For this reason, in the case of a tension mask cathode ray tube, another characteristic value is required in order to select the inner shield material in addition to the magnetic permeability xcexc, which is the characteristic value used in the formed mask cathode ray tube.
U.S. Pat. No. 5,871,851 discloses that the value of multiplying the coercive force Hc by the residual magnetic flux density Br is taken as the characteristic value for discriminating the desired inner shield material. With the inner shield formed using a material bearing the predetermined specific value (Hcxc3x97Br), the movement of the geomagnetic field can be minimized in the tension mask cathode ray tube. Preferably, the specific value of Hcxc3x97Br is established to be 28 or more.
However, according to the experiments of the present inventor, as shown in FIG. 2, in the case the specific value of Hcxc3x97Br is 60 or more, the movement scale of the electron beam is reduced with the increase in the specific value of Hcxc3x97Br. By contrast, in the case the specific value of Hcxc3x97Br is 80 or more, the movement scale of the electron beam is rather enlarged. In FIG. 2, the solid line indicates the value of 0.5(NS+EW), and the dotted line indicates the value of NS+EW.
As indicated in FIG. 2 by the solid line, the value of 0.5(NS+EW) indicates the value of NS+EW at the location where the target moves by xc2xd the distance vertically proceeding from the diagonal end of the cathode ray tube to the horizontal center axis of the screen. In the case of a formed mask cathode ray tube, the value of NS+EW exhibits the maximum value at the diagonal area. By contrast, in the case of a tension mask cathode ray tube, the movement of the geomagnetic field exhibits the maximum value at the location where the target moves by xc2xd of the distance vertically proceeding from the diagonal end of the cathode ray tube to the horizontal center axis of the screen.
Accordingly, the specific value of Hcxc3x97Br disclosed in U.S. Pat. No. 5,871,851 is not effective in selecting the inner shield material.
It is an object of the present invention to provide a tension mask cathode ray tube with a geomagnetism-shielding inner shield which minimizes variation in the landing position of the electron beam due to the geomagnetic field.
This and other objects may be achieved by a tension mask cathode ray tube. The tension mask cathode ray tube is provided with a tension mask for color-selecting electron beams emitted from an electron gun, and a mask frame holds the tension mask such that the tension mask is tensioned in a uni-axial direction or in bi-axial directions. An inner shield is fixed to the mask frame to shield geomagnetism. A specific value Br/Hc of the inner shield where Br indicates the residual magnetic flux density and Hc indicates the coercive force is established to be 1.0-2.0 times more than the specific value Br/Hc of the mask frame, or to be 1.0-2.5 times more than the specific value Br/Hc of the tension mask. Alternatively, the specific value Br/Hc of the inner shield may be established to be 1.0-2.0 times more than the specific value Br/Hc of the mask frame while being 1.0-2.5 times more than the specific value Br/Hc of the tension mask.
The mask frame is formed with a material bearing a coercive force Hc of 3.0 Oe or more. With the tension mask cathode ray tube, a tensional strength of 15 kgf/mm2 or more should be applied to the tension mask. Particularly, the cathode ray tube should bear a reasonable rigidity at a high temperature of 450xc2x0 C. or more during the steps of blackening and sealing. Therefore, a material bearing a high temperature-resistant rigid material should be used to form the mask frame. Such a material involves a higher carbon content and a smaller grain size, and hence, the coercive force Hc thereof reaches 3.0 Oe or more. In the case a material bearing a coercive force Hc of less than 3.0 Oe is used to form the mask frame, a predetermined tensional strength required for the tension mask cathode ray tube cannot be applied to the tension mask.
The tension mask is formed with a material bearing a magnetic permeability of 300 or less at 350 mG. The tension mask should endure a tensional force of 15 kgf/mm2 or more, and a reasonable rigidity at a high temperature of 450xc2x0 C. or more during the step of blackening. For that purpose, a full hard material is usually introduced that can bear a coercive force Hc of 3.0 Oe or more, with an initial magnetic permeability of 300 or less (the permeability at 350 mG). In the case a material bearing an initial permeability of more than 300 is used to form the mask, a predetermined degree of tensional strength cannot be applied to the mask. The tension mask may have a thickness of 0.05-0.20 mm.
With the formed mask cathode ray tube, a material with a thickness of 0.15 mm has been extensively used to form the inner shield. However, with the tension mask cathode ray tube, the weight of the mask and the mask frame reaches 6 kg or more, which is two times more than that of the formed mask cathode ray tube. The inner shield because it has a thickness of 0.20-0.50 mm and a predetermined degree of strength must endure the weight of the mask and the frame during the process of manufacturing the cathode ray tube.
The tension mask is tensioned with a tensional strength such that the degree of strength at the periphery of the tension mask is greater than the degree of strength at the center of the tension mask, or the degree of strength at the center of the tension mask is greater than the degree of strength at the periphery of the tension mask. The cathode ray tube further includes a vacuum tube having a panel with a phosphor screen, a funnel sealed to the panel, and a neck connected to the funnel. An electron gun is mounted within the neck, while a deflection yoke deflects the electron beams emitted from the electron gun.